1. Field of Invention
The present invention relates to electro-optical devices capable of performing gray-scale display, to methods and circuits for driving the same, and to electronic apparatuses.
2. Description of Related Art
Electro-optical devices, such as liquid crystal displays using liquid crystal as an electro-optical material, are widely used as display devices in place of cathode-ray tubes (CRTs) in displays of various information processing apparatus, wall-mounted televisions, and the like.
By way of example, a conventional electro-optical device has the following structure. Specifically, the conventional electro-optical device typically includes a device substrate on which pixel electrodes aligned in the form of a matrix and switching devices, such as TFTs (Thin Film Transistors) connected to the pixel electrodes, are provided, an opposing substrate on which counter electrodes opposed to the pixel electrodes are formed, and liquid crystal, i.e., electro-optical material, filled between the two substrates. With this arrangement, when a scanning signal is supplied to the switching devices via scanning lines, the switching devices become conducting. In this conducting state, when an image signal with a voltage, in accordance with a gray level, is supplied to the pixel electrodes through data lines, a charge in accordance with the voltage of the image signal is accumulated in a liquid crystal layer between the pixel electrodes and the counter electrodes. When the switching devices enter an off-state, after the charge has been accumulated, the accumulated charge in the liquid crystal layer is maintained by the capacitance of the liquid crystal layer and by storage capacitors. Accordingly, when the switching devices are driven so as to control the amount of charge that is accumulated in accordance with the gray level, alignment of the liquid crystal varies according to each pixel, that is, the gray level varies according to each pixel. As a result, gray-scale display can be performed.
It is only necessary to accumulate charge in the liquid crystal layer of each pixel for a partial period. First, a scanning-line driving circuit sequentially selects each scanning line. Second, a data-line driving circuit sequentially selects each data line within the scanning-line selection period. Third, an image signal with a voltage in accordance with a gray level is sampled on the selected data line. As a result, time-division multiplexing driving in which the scanning line and the data line are shared by a plurality of pixels is made possible.
An image signal supplied to the data line is a voltage in accordance with the gray level, that is, an analog signal. It is necessary to provide a D/A converter circuit and an operational amplifier in peripheral circuits of the electro-optical device. This causes an increase in the cost of the overall device. In addition, display unevenness is caused by nonuniformity in characteristics of the D/A converter circuit and the operational amplifier and by nonuniformity in various wiring resistances. It is therefore difficult to perform high-quality display. In particular, this problem becomes noticeable in performing high-definition display.
In the above-described conventional electro-optical device, it is necessary to apply the image signal to all pixels at predetermined time intervals. Specifically, it is necessary to sequentially select all the scanning lines by the scanning-line driving circuit at predetermined time intervals. Also, it is necessary to apply the image signal with a voltage in accordance with the gray level, to all data lines, in every selection period. As a result, the power consumption is increased. In particular, this problem becomes noticeable when the number of pixels to which the image signal is applied, is increased in order to enhance the resolution. Although various techniques have been implemented in order to reduce the power consumption, it is still necessary to apply the image signal to all the pixels at predetermined time intervals. In the present circumstances, there is a limit to lowering the power consumption.